Surface flashover resistant capacitors and method for producing same

ABSTRACT

A surface flashover resistant multilayer ceramic capacitor. The capacitor has a plurality of layers of dielectric material and a plurality of electrodes disposed between the layers of dielectric material. End caps are located at either end of the capacitor and are connected to one or more of the internal electrodes. A coating comprising one or more insulative layers is applied to the outer surface of the capacitor and selected portions of the coating are subsequently removed. The coating of insulative layer comprises a polymer, and specifically a poly-para-xylylene. The insulative coating is applied through a vapor deposition process. The selected portions of the insulating layer are removed by laser ablation.

FIELD OF THE INVENTION

[0001] The present invention relates to electrical capacitors and moreparticularly to improved ceramic capacitors able to withstand surfaceflashover and a method for the manufacture of these devices.

BACKGROUND OF THE INVENTION

[0002] Ceramic capacitor technology covers a wide range of product typesbased on a multitude of dielectric materials and physicalconfigurations. Regardless of the particular composition of a capacitor,all such devices are capable of storing electrical energy and find manyapplications in the field of electronics including: discharging storedenergy, blocking direct current, coupling AC circuit components,bypassing AC signals, discriminating among frequencies, suppressingtransient voltages, etc.

[0003] Multilayer ceramic capacitors are available in two generalconfigurations. They are sold as bare leadless (chip) components, orencapsulated leaded devices. Traditionally, the chip version has beenused in densely packed hybrid and delay line circuits, while the leadedcapacitor has dominated the high volume printed circuit board market,which is tooled for the automatic insertion of axial or radial leadcomponents of all types. The need for higher packing densities ofcomponents on printed circuit boards led to the development of surfacemount technology which involves high speed automatic placement ofleadless components. Components destined for surface mount are usuallypacked in tape and reel format and subsequently fed into placementmachines that remove individual components from the tape and tack themto the surface of the printed circuit board with a non-conductive epoxy.Subsequent electrical attachment to conductive sites on the printedcircuit board is typically accomplished with traditional solder waveprocessing. The greatly expanded use of surface mount technology hasdramatically increased the importance of the physical size of surfacemount components, such as multilayer ceramic chip capacitors. Smallercomponents yield higher component densities on the printed circuitboards and in turn smaller electronic devices. Thus, reducing the sizeof surface mount components is of great import.

[0004] The body of a multilayer ceramic capacitor is composed ofalternating layers of ceramic dielectric material and conductiveelectrodes. The chip version of the device is completed by the additionof a pair of external conductive end caps or terminals placed atopposite ends of the body of the device. The leaded version of thedevice begins with the same multilayer ceramic and metal composite bodyas the chip version of the component but is completed by the addition ofa pair of protruding conductive leads attached to opposite ends of thebody and a nonconductive layer. The nonconductive layer is applied tothe entire external surface of the body and leads with the exception ofthose portions of the leads which extend beyond the body (see e.g., U.S.Pat. No. 5,888,590).

[0005] Surface flashover is a common problem associated with ceramicchip capacitors. It represents a failure of the component and maydestroy the component itself or damage electronic equipment of which thecomponent is a part. Surface flashover is characterized by an electricalarc between the metal end caps that travels across the external surfaceof the outermost layer of ceramic dielectric material. The distancebetween the metal end caps and the voltage across the capacitor are thedominant factors in determining whether surface flashover will occur.Some other characteristics of the capacitor that may affect thesize/voltage level at which surface flashover will occur include:surface contamination, properties inherent to the ceramic dielectricmaterial, and polarization within the ceramic dielectric material.Although insulative coatings could help to alleviate surface flashover,known applications are relatively bulky in comparison to the size ofceramic chip capacitors and may impede the ability of placement machinesto handle such parts during high-speed surface mount operations.Additionally, the conductive end caps of multilayer ceramic chipcapacitors must be exposed in order for them to be attached by solderingto a printed circuit board. Preventing the coating of these end capswhile facilitating the coating of the remainder of the chip capacitor isdifficult given the nature of current capacitor coating applicationprocesses and the relatively small size of the end caps that must remainfree of the coating material.

[0006] “Parylene” is a general term used to describe a class ofpoly-para-xylylenes which are derived from a dimer having the structure:

[0007] wherein X is typically hydrogen or a halogen. Common forms ofparylene dimers include the following:

[0008] Parylene films are formed from their related dimers by means of awell-known vapor deposition process in which the dimer is vaporized,pyrolized and passed into a deposition chamber, wherein the monomermolecules deposit and polymerize onto the contents of the depositionchamber according to the following reaction:

[0009] Parylene films (see e.g., U.S. Pat. No. 4,500,562) are well knownin the electronic arts and are typically employed due to their abilityto conform to items with varied geometries and withstand environmentalconditions. For example, they have been used to protect electronicdevices, sensors and batteries from adverse environmental conditions(see U.S. Pat. Nos. 6,138,349, 3,676,754 and 5,561,004 respectively).Parylene films have also been employed to insulate wire leads to preventshort-circuits when the leads are physically deformed (see U.S. Pat. No.5,656,830), to form internal dielectric layers in capacitors embedded inprinted circuit boards (U.S. Pat. No. 6,068,782) and as a portion of theinternal dielectric layer of discrete capacitors (U.S. Pat. Nos.3,333,169 and 3,397,085). One useful way to form these films is viavapor deposition (see e.g., U.S. Pat. Nos. 5,534,068, 5,536,319,5,536,321, 5,536,322).

[0010] It is desirable for a ceramic chip capacitor, which will beemployed as a safety capacitor, to be as small as possible yet able towithstand high voltage levels. Typically such parts are certified toindustry or international standards to ensure reliability. Thesestandards define, among other things, the voltage level that a capacitorof a given physical size must be able to withstand. For example, onesuch standard defines a ceramic chip capacitor measuring 0.18 inches inlength able to withstand 2700 VDC and another defines a ceramic chipcapacitor measuring 0.22 inches in length able to withstand 5000 VDC.

SUMMARY OF THE INVENTION

[0011] The present invention is directed to a surface flashoverresistant multilayer ceramic capacitor. The capacitor has a plurality oflayers of dielectric material and a plurality of electrodes disposedbetween the layers of dielectric material. End caps are located ateither end of the capacitor and are connected to one or more of theinternal electrodes. A coating comprising one or more insulative layersis applied to the outer surface of the capacitor and selected portionsof the coating are subsequently removed. The coating of insulative layercomprises a polymer, and specifically a poly-para-xylylene. Preferably,the insulative coating is applied through a vapor deposition process.The selected portions of the insulating layer are removed preferably bylaser ablation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1A is a cross sectional view of a coated capacitor in accordwith one embodiment of the present invention.

[0013]FIG. 1B is a cross sectional view of a coated capacitor in accordwith an alternative embodiment of the present invention.

[0014]FIG. 1C is a cross sectional view of a coated capacitor in accordwith another alternative embodiment of the present invention.

[0015]FIG. 2 is an exemplary process by which the capacitors of FIGS.1A-1C are formed in accord with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The capacitors of the present invention are shown in FIGS. 1A-1Cwhich illustrates three cross-sectional views of a coated multilayerceramic capacitor according to the present invention. The capacitorcomprises a body, the body further comprising a plurality of layers ofdielectric material 10 and a plurality of internal electrodes 11. Thecapacitor further comprises a pair of conductive end caps 12 placed atopposite ends of the body with each end cap electrically connected toone or more of the internal electrodes. The capacitor further comprisesan insulative coating 13.

[0017] The dielectric material 10 may be any suitable material such asmica, glass, alumina, titania, barium titanate, a formulated ceramic orother material having dielectric properties. Though capacitors thatcontain dielectric materials formed from these substances derive benefitfrom the application of an insulative coating material, more benefit maybe realized by devices that have exposed ceramic dielectric materialsand closely spaced conductive terminals. Prior to the application of theinsulative coating according as described below, uncoated ceramic chipcapacitors are prone to the surface flashover phenomenon describedabove. An insulative coating layer covering the body of the capacitor orthe body and selected portions of the conductive end caps enhances thecapacitor's ability to tolerate applied voltages. Generally, a capacitorhaving an insulative coating will be able to withstand higher voltagelevels without experiencing surface flashover than an otherwiseidentical capacitor without an insulative coating.

[0018] The internal electrodes 11 and conductive end caps 12 and 14 maybe formed of any suitable material but will typically be formed of aconductive metal. For example, a silver and palladium alloy is commonlyused to form both the internal electrodes 11 and the conductive end caps12 and 14. The internal electrodes 11 are typically formed by a screenprinting process in which the conductive material is applied to aceramic layer in a manner that will allow its electrical connection toonly one end caps 12 or 14. An additional ceramic layer is applied abovethe screen printed layer and the process is repeated as many times asdesired to with the capacitance of the resulting device beingproportional to the number of layers. The sheets of layered conductiveand ceramic material are then cut into individual pieces that comprisethe body of the capacitor. Opposite ends of the body are then dippedinto a conductive thick film paste to form the conductive end caps 12and 14.

[0019] In one embodiment of the present invention a primer 18 (FIG. 2)is applied to the entire outer surface of the body of the capacitor andselected portions of the conductive end caps 12 and 14. The primer aidsin the deposition insulative coating 13 later applied to the outersurface of the capacitor. A number of suitable primers are known in theart. For example, a primer from a group of chemicals collectively knownas silane will aid in the deposition of certain polymers. One embodimentof the present invention contemplates dipping 19 (FIG. 2) the capacitorbodies in such a primer before applying the insulative coating 13.

[0020] The present invention includes an insulative coating 13 appliedto the body of the capacitor and selected portions of the conductive endcaps 12 and 14. In one embodiment of, the insulative coating 13 iscomprised of a material chosen from the group of materials collectivelyreferred to by the trade name parylene. Parylene coating materials arecommercially available as dimers and include parylene N(polyparaxylylene), parylene C (monochloropolyparaxylylene) and paryleneD (dichloropolyparaxylylene). Each of these materials is suitable foruse in the coating 13 of the present invention. The first embodiment ofthe present invention employed parylene C as the coating material.

[0021] As shown in FIG. 2, application of the coating begins with theoptional step of applying 19 a primer to the entire outer surface of thecapacitor. The remainder of the insulative coating is typically appliedby a vapor deposition method such as shown in FIG. 2. According to thefirst step of the vapor deposition process, an insulative coatingmaterial, for example the parylene dimer 20, is vaporized by thevaporizer 21. In the case of a parylene dimer, the vaporization willtypically occur at about 150 degrees C. This is followed by quantitativecleavage of the dimer at about 680 degrees C. in the pyrolytic chamber23 to yield the stable monomeric diradical, para-xylylene 22. Themonomer then enters a roughly room temperature deposition chamber 25where it simultaneously adsorbs and polymerizes on the capacitor forminga parylene polymer 24. The parts typically remain in the depositionchamber until a coating 13 of a suitable thickness has formed on theouter surface of each part. Deposition via this process is advantageousin that the capacitor is not placed under thermal stress as it neverrises more than a few degrees above ambient.

[0022] Next, selected portions of the insulative coating 13 are removed.In one embodiment of the present invention, selected portions of thecoating 13 are removed by laser ablation 27 to expose a portion (FIG.1B) or all (FIG. 1C) of the conductive end caps following the vapordeposition of the insulative coating, Coated capacitors are fed from avibratory bowl 28 onto a sheet 30 where they are held in a verticalposition. The sheet moves beneath a stationary laser 32 which ablatesselected portions of the coating 13 from the exposed end cap 12 or 14.The capacitors are then reoriented in such a manner that the previouslyunexposed end cap 12 or 14 is now exposed. Selected portions of theconductive coating are then removed from the newly exposed conductiveend cap.

[0023] Although the present invention is described and illustrated withrespect to the embodiments and method described herein, it is to beunderstood that the invention is not to be limited thereto since changesand modifications can be made without departing from the scope of theinvention as hereinafter claimed.

What is claimed is:
 1. A multilayer ceramic capacitor comprising: a bodyhaving a plurality of layers of dielectric material and a plurality ofelectrodes disposed among the layers of dielectric material, the bodyhaving an outer surface, a first end and a second end opposite the firstend; a first end cap located at the first end of the body and a secondend cap located at the second end of the body, each end cap connected toone or more of the internal electrodes; a coating comprising one or moreinsulative layers applied to the entire outer surface of the capacitorexcept selected portions of the conductive end caps.
 2. The capacitoraccording to claim 1 wherein at least one of the insulative layerscomprises a polymer.
 3. The capacitor according to claim 1 wherein atleast one of the insulative layers comprises poly-para-xylylene.
 4. Thecapacitor according to claim 1 wherein the dielectric material comprisesa ceramic.
 5. The capacitor according to claim 4 wherein the coatingfurther comprises a primer material.
 6. The capacitor according to claim1 wherein the capacitor is up to about 0.18 inches long.
 7. Thecapacitor according to claim 6 wherein the capacitor can withstand a DCvoltage of at least about 3000 Volts.
 8. The capacitor according toclaim 6 wherein the capacitor can withstand a DC voltage of at leastabout 5000 Volts.
 9. A multilayer ceramic chip capacitor comprising: aplurality of layers of dielectric material; a plurality of electrodesdisposed among the layers of dielectric material; a plurality ofconductive terminals, each terminal connected to one or more of theconductive layers; and means for preventing surface flashover comprisinga conformal poly-para-xylylene coating formed on an outer surface of thecapacitor except selected portions of the conductive terminals.
 10. Thecapacitor according to claim 9 wherein the conformal poly-para-xylylenecoating is formed by vapor deposition.
 11. The capacitor according toclaim 10 wherein the capacitor is up to about 0.18 inches long.
 12. Thecapacitor according to claim 11 wherein the capacitor can withstand a DCvoltage of at least about 300 Volts.
 13. The capacitor according toclaim 12 wherein the means for preventing surface flashover alsoincludes a primer.
 14. The capacitor according to claim 11 wherein thecapacitor can withstand a DC voltage of at least about 5000 Volts.
 15. Amethod for making a surface flashover resistant capacitor comprising:forming an insulative coating on an outer surface of the capacitor, thecapacitor comprising one or more dielectric layers, a plurality ofinternal electrodes disposed among the dielectric layers, a pair ofconductive terminals with each conductive terminal connected to one ormore internal electrodes; and selectively removing portions of theinsulative coating from the conductive terminals.
 16. The method ofclaim 15 wherein selectively removing portions of the insulative coatingcomprises laser ablation of the selected portions.
 17. The method ofclaim 15 wherein forming an insulative coating comprises vapordeposition of a substance.
 18. The method of claim 17 wherein thesubstance comprises a polymer.
 19. The method of claim 18 wherein thepolymer comprises para-poly-xylylene.
 20. A method for making a surfaceflashover resistant capacitor comprising: applying a primer layer to anouter surface of the capacitor, the capacitor comprising one or moredielectric layers, a plurality of internal electrodes disposed among thedielectric layers, a plurality of conductive end caps with eachconductive end cap connected to one or more internal electrodes; formingan insulative coating on the primer layer; and selectively removingportions of the insulative coating and primer layer from the conductiveterminals.
 21. The method of claim 20 wherein the coating is formed byvapor deposition.
 22. The method of claim 20 wherein selectivelyremoving portions of the insulative coating and primer layer compriseslaser ablation of the selected portions.
 23. The method of claim 20wherein forming an insulative coating comprises vapor deposition of asubstance.
 24. The method of claim 23 wherein the substance comprises apolymer.
 25. The method of claim 24 wherein the polymer comprisespara-poly-xylylene.